title: Structure and hydration of the M-state of the bacteriorhodopsin mutant D96N studied by neutron diffraction.
abstract: Neutron diffraction from oriented purple membrane fragments at various hydration levels, coupled with H2O/2H2O exchange, was used to compare the structure and hydration of the light-adapted initial state (B-state) and the M photointermediate of bacteriorhodopsin mutant D96N. Diffraction patterns were recorded at 86%, 75% and 57% relative humidity (r.h.). Structural changes observed at 86% and 75% r.h. are absent at 57% r.h., showing that they are uncoupled from the deprotonation of the Schiff base during formation of the M-state. In a current model, the M-state consists of two substates, M1 and M2. Our data suggest that the state trapped at 57% r.h. is M1 and that M2 is trapped at the higher r.h. values. The observed structural changes are, therefore, associated with the M1-->M2 transition, which can only take place at higher r.h. The difference Fourier projections of exchangeable hydrogen atoms and water molecules in the membrane plane are very similar for the B and M-states at 75% and 86% r.h. This shows that contrary to certain models, the structural changes in the M-state are not correlated with major hydration changes in the proton channel projection.
authors.affiliation: Institut de Biologie Structurale, Jean Pierre Ebel CEA-CNRS, Grenoble, France.
authors.forename: M
authors.initials: M
authors.lastname: Weik
authors.forename: G
authors.initials: G
authors.lastname: Zaccai
authors.forename: N A
authors.initials: NA
authors.lastname: Dencher
authors.forename: D
authors.initials: D
authors.lastname: Oesterhelt
authors.forename: T
authors.initials: T
authors.lastname: Hauss
mesh_terms: D001216:Asparagine / Q000235:genetics; D001224:Aspartic Acid / Q000235:genetics; D001436:Bacteriorhodopsins / Q000235:genetics* / Q000378:metabolism*; D005583:Fourier Analysis; D019617:Halobacterium salinarum; D009154:Mutation; D009502:Neutrons; D018717:Purple Membrane / Q000737:chemistry / Q000378:metabolism; D012545:Schiff Bases / Q000737:chemistry; D014867:Water / Q000378:metabolism*; D014961:X-Ray Diffraction
chemical_list: D012545:Schiff Bases; D014867:Water; D001224:Aspartic Acid; D001436:Bacteriorhodopsins; D001216:Asparagine